Green’s Dictionary of Slang

(Aus., usu. prison) a sexual pervert.

1949		R. Park Poor Man’s Orange 38: ‘Just look at that dirty ole secko, will you?’ he said disgustedly, and scooping up a stone he ran after it, yelling, ‘Merv, Merv, the rotten old perv.’.
1967		‘Whisper All Aussie Dict.’ Kings Cross Whisper (Sydney) xxxix 4/5: sexos or seckos: Prisoners sentenced for sex crimes against young persons. The most despised of all prisoners.
1977		C. Hope Ducktails in Gray Theatre Two (1981) 44: bo-bo Hell, I fancy that dark-haired goosey. [...] D’you check the way she sort of looked at me? I think I’m in there. howellsie Kap it uit, ou sekkie!
1984		B. Ellem Doing Time 195: secco (or secko): someone who has been sentenced for a sexual offence; or someone who is obsessed with sex.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Secco. A sexual offender. Term generalises to someone who is obsessed with sex.
1999		G. Seal Lingo 77: Some less than complimentary examples include secko for a sex maniac or pervert, also sicko for someone whose interests are off-centre.
2006		B. Matthews Intractable [ebook] Lawson was a liferand a serious secko.